Leucine aminopeptidase, isolated in crystalline form from the lens tissue of the ox eye, is the subject of a number of investigations designed to characterize the enzyme and determine its mechanism of action with a view to clarify the possible role of the enzyme in cataract formation. The role of various divalent metals in the action of the enzyme will be investigated by binding studies, effect on enzymatic activity and spectral properties and correlated with the presence of the metals in normal and cataractous lenses. Kinetic studies will be performed on a variety of substrates to determine the size of the substrate binding site(s). Active-site-directed irreversible inhibitors will be sought to aid in the elucidation of the mechanism of catalysis. The modification of functional groups will be correlated with effects on enzymatic activity and structure. The quaternary structure and certain elements of the primary, secondary and tertiary structure of the enzyme will be investigated.